Corrected the twitter and @excon links
Corrected the twitter and @excon links

Non-coding section of the contributing guide had markdown typos [ci skip]
mahemoff/excon,ilackarms/excon,liftforward/excon,wadetandy/excon,TheNeatCompany/excon,excon/excon,hammerdr/excon,cloudfoundry-incubator/excon